Grey lit screen and no keyboard input - is it the motherboard?

Hi,

I have an old 2007 macbook. A while ago now I started to have to boot the computer using safe boot due to corrupted security permissions, all would work well for abfew weeks then it would need it again.

Finally all I started to get was a grey screen on startup.

I tried to reinstall OSX from dvd but this resulted in the dvd getting stuck in the drive which after none of the usual and unusual solutions worked led to me having to dismantle the drive.

I figured that the HDD which is the original had failed but replacing the drive has made no difference so this is how the computer boots at the moment:

1) Grey screen

2) Power lights work,

3) Keyboard lights don't come on - keyboard keys appear not to work (e.g c for dvd boot etc.)

4) External drives if plugged in get power.

5) External usb keyboards plugged in don't light up either, nor do their keys appear to affect boot.

6) I had the boot chime muted before all this happened so I don't get a chime and couldn't say if it were playing or not.

7) The new internal hdd and old dvd drives do sound normal.

Could this be the ram (I don't think I have any to test with) or is it more likely the motherboard? Any ways to get the external keyboard working?

I can't really throw money at the computer at the moment so trying to rule out as much as possible.

Thanks.